Why do these conclusions continue to be drawn by biblical scholars, as if the canonical gospels are any more accurate (or "peerless") theologies and histories than the non-canonical gospels? All these texts are theologies, and whether or not they are "peerless" depends upon where you are standing. None of our texts are histories, let alone accurate histories. And how much historical information we can actually reap out of any of them, and the procedures for doing so, are questions more problematic than not.
As for the accuracy of the Church Fathers' descriptions. Their accuracy is not how I frame any discussion of a normative debate. The Church Fathers passed on false information, ill-informed interpretations, and fabricated stories in their struggle against those forms of Christianity that they hated. As the old saying goes, "All is fair in love and war."
As scholars, it is our job to stop buying into the normative rhetoric, and figure out what was really going on on the ground. The Gospel of Judas helps us tremendously in this venture. We can see that it was not connected with Cain or the Cainites as some of the Church Father testimony suggests. It is written by Sethians, for whom Cain was an evil Archon! The evidence from the Gospel of Judas leads me to conclude that the Cainites were a fantasy of the Church Fathers, a result of their war to become the dominant form of Christianity.